Creating light has always fascinated man. Great efforts have been made to develop and improve different light sources.
One interesting type of light sources is the fluorescent tube. It has many advantages, but suffers from serious drawbacks. For example, there is always a delay after the power has been turned on until it starts to operate giving full light. It needs complicated control equipment, which requires space. To obtain light with a source of this kind it is unfortunately necessary to use materials having negative environmental effects.
Cathodoluminescent light sources is also an interesting type of light sources. Such light sources, including an evacuated envelope containing a grid and a heated cathode, for emission of electrons, are known from GB, A, 2 070 849 (The General Electric Company Limited), GB, A, 2 097 181 (The General Electric Company PLC), GB, A, 2 126 006 (The General Electric Company plc) and GB, A, 2 089 561 (The General Electric Company Limited). The insides of the envelopes are covered with a layer of phosphor of an electron-responsive type. These cathodoluminescent lamps have essentially the form of an electric bulb.
Since these light sources all have a heated cathode, the cathode has to be heated by special means, before the emission of light starts.
The use of electrons exciting phosphor to luminescence has the effect that more heat is produced than in comparable fluorescent tubes. It is therefore advantageous if the active surface, for the emission of light and for the necessary heat dissipation, is large. The cathodoluminescent lamps shown in the documents mentioned do not have optimal surfaces.
A light source of the kind mentioned in the first paragraph above is disclosed in U.S. Pat. No. 5,588,893 (Kentucky Research and Investment Company Limited). A field emission cathode is arranged inside an evacuated glass container having a luminescent layer arranged on its inner surface. A modulator is provided between the cathode and the luminescent layer. The cathode includes carbon fibers, arranged in bundles, preferably in a matrix, on a substrate. This light source is considered to be the prior art closest to the invention concerning a light source. The content of U.S. Pat. No. 5,588,893 is incorporated herein by reference.
However in the last-mentioned known light source, electrons are emitted only in a direction perpendicular to the substrate. Also there is no indication in the document how to produce the light source in a cost-efficient way.
The above mentioned U.S. Pat. No. 5,588,893 (Kentucky Research and Investment Company Limited) also discloses a field emission cathode of the kind mentioned above. The cathode disclosed includes carbon fibers, arranged in bundles, preferably in a matrix, on a substrate. The document also discloses a method including treatment of the emitting surfaces in order to achieve a cathode with higher efficiency than previous cathodes. This cathode is considered to be the prior art closest to the invention concerning a cathode.
Further, DE, C2, 40 02 049 (Deutsche Forschnungsanstalt fur Luft- und Raumfahrt e.V.) discloses an electron emitting source including a cathode which comprises small, felted or fabric plates, spaced apart from each other. The plates can consist of felted carbon fibers, and be arranged on a cylindrical cathode body The use is for irradiating a medium with electrons.
U.S. Pat. No. 4,272,699 (Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V.) discloses a field emission cathode in an electron impact ion source for an instrument such as a mass spectrometer or molecular beam detector. The cathode has angular configuration, and includes bundles of carbon fibers, with their emitting surfaces directed inwards.
Previously known field emission cathodes are often of a complicated and fragile construction, especially as concerns the mountings and the attachment of field emitting bodies.
It has been found in connection with cathodes including fibers that the electrical fields acting between the cathode and a grid or an anode will cause individual fibers to get loose from their carrier if they are not safely secured thereto. Once loose, the fibers will, in most cases, be attracted by the grid and cause a short circuit between the cathode and the grid, until it burns oft after some time due to the resulting current through the fiber.